Add HintPane or popup hints to not obvious features that go away when those features are used

[tomcl]

Describe the solution you'd like
Ctrl-left-button down on empty canvas should drag the schematic.

Additional context

• We use left-mouse down to select a component or rectangle. Ctrl-left-button down us used to move ports or resize components. It makes sense also to use this to drag the window.
• Do we still need the schematic scroll bars?

[tomcl]

It turns out we had Shift-drag working anyway. I have added Ctrl-drag.

There is an issue with features not being obvious. We could maybe add some novice-user hints linking them to behaviour so that once features are used the hints go away. We have per-PC per-user stored data so this is possible.

Note that we do this already with HintPane and the hint to use the custom-component resizing UI.

I will keep this open for possible work on hints.